A blindstich sewing machine produces a chainstitch which has the inherent disadvantage that it easily unravels. Thus, there is an advantage in providing a blindstich sewing machine with means for effecting a tacking or locking operation at the end of the sewing operation, thus preventing the seam from unraveling. By reason of the special and unique construction of a blindstitch sewing machine, conventional arrangements for stitch shortening or tacking that have been used on other types of machines cannot be transferred for this purpose to blindstitch sewing machines.
A blindstitch sewing machine with a tacking or seam locking means is disclosed in the U.S. Pat. No. 4,114,547, issued to R. Russell. This patented apparatus accomplishes the tacking effect by providing a device which lowers the work support without lowering the ridge former. By lowering the work support during sewing, there will be no feeding effect on the workpiece and, thus, a series of stitches may be secured in the workpiece within a relatively short distance. A number of stitches formed in generally the same area, will provide a locking effect on the seam. The disadvantage of the patented apparatus is that the workpiece is inadequately clamped when the work support is lowered. Thus, the needle may fail to pierce the workpiece and, thus, an unacceptable garment may result.